The present invention relates to a vehicle attitude control apparatus whereby the behaviour of a vehicle can be stabilized.
If a vehicle has entered a state of under-steer or a state of over-steer, then it is sought to stabilize the behaviour of the vehicle by controlling the braking forces applied to the vehicle wheels.
However, if the steering angle is raised excessively in order to eliminate a state of under-steer, then it becomes difficult to maintain a linear region where the lateral slip angle of the vehicle wheels is proportional to the cornering force, and hence even if the braking forces are controlled in the foregoing manner, it is not possible to stabilize the vehicle behaviour.
It is an object of the present invention to provide a vehicle attitude control apparatus capable of resolving the aforementioned problem.
The vehicle attitude control apparatus according to the present invention comprises: an operating member; a steering actuator driven in accordance with the operation of the operating member; a mechanism for transmitting movement of the steering actuator to the wheels of a vehicle, in such a manner that the steering angle changes in accordance with the movement thereof; means for determining a behaviour index value corresponding to changes in vehicle behaviour on the basis of changes in the steering angle; means for determining the amount of operation of the operating member; means for determining a target behaviour index value corresponding to the determined amount of operation, on the basis of a stored relationship between the amount of operation and the target behaviour index value; means for determining whether or not the vehicle is in a state of under-steer; means for controlling the steering actuator in such a manner that the behaviour index value follows the target behaviour index value, when the vehicle is not in the state of under-steer; means for controlling the braking forces at the vehicle wheels in such a manner that the behaviour index value follows the target behaviour index value, when the vehicle is not in the state of under-steer; and means for determining an instability index value correlating to the degree of under-steer.
A first aspect of the present invention further comprises: means for storing a predetermined first relationship between the behaviour index value, the target behaviour index value, the instability index value and a corrected behaviour index value for steering; means for determining the corrected behaviour index value for steering on the basis of the determined behaviour index value, the determined target behaviour index value, the determined instability index value, and the stored first relationship; means for storing a predetermined second relationship between the behaviour index value, the target behaviour index value, the instability index value and a corrected behaviour index value for braking; and means for determining the corrected behaviour index value for braking on the basis of the determined behaviour index value, the determined target behaviour index value, the determined instability index value, and the stored second relationship; wherein the first relationship is determined in such a manner, as the magnitude of the instability index value increases up to a predetermined set value, the deviation between the target behaviour index value and the corrected behaviour index value for steering decreases, and when the target behaviour index value and the behaviour index value are equal, the corrected behaviour index value for steering becomes equal to the behaviour index value; the second relationship is determined in such a manner that as the magnitude of the instability. index value increases up to a predetermined set value, the deviation between the target behaviour index value and the corrected behaviour index value for braking increases, and when the target behaviour index value and the behaviour index value are equal, the corrected behaviour index value for braking becomes equal to the behaviour index value; and when the vehicle is in the state of under-steer, the steering actuator is controlled in such a manner that the corrected behaviour index value for steering instead of the behaviour index value follows the target behaviour index value, and the braking forces are controlled in such a manner that the corrected behaviour index value for braking instead of the behaviour index value follows the target behaviour index value.
A second aspect of the present invention comprises: means for storing a predetermined first relationship between the behaviour index value, the target behaviour index value, the instability index value and a corrected target behaviour index value for steering; means for determining the corrected target behaviour index value for steering on the basis of the determined behaviour index value, the determined target behaviour index value, the determined instability index value, and the stored first relationship; means for storing a predetermined second relationship between the behaviour index value, the target behaviour index value, the instability index value and a corrected target behaviour index value for braking; and means for determining the corrected target behaviour index value for braking on the basis of the determined behaviour index value, the determined target behaviour index value, the determined instability index value, and the stored second relationship; wherein the first relationship is determined in such a manner, as the magnitude of the instability index value increases up to a predetermined set value, the deviation between the corrected target behaviour index value for steering and the behaviour index value decreases, and when the target behaviour index value and the behaviour index value are equal, the corrected target behaviour index value for steering becomes equal to the target behaviour index value; the second relationship is determined in such a manner that, as the magnitude of the instability index value increases up to a predetermined set value, the deviation between the corrected target behaviour index value for braking and the behaviour index value increases, and when the target behaviour index value and the behaviour index value are equal, the corrected target behaviour index value for braking becomes equal to the target behaviour index value; and when the vehicle is in the state of under-steer, the steering actuator is controlled in such a manner that the behaviour index value follows the corrected target behaviour index value for steering, instead of the target behaviour index value, and the braking forces are controlled in such a manner that the behaviour index value follows the corrected target behaviour index value for braking instead of the target behaviour index value.
According to each of the aspects of the present invention, when the vehicle is not in a state of under-steer, the steering actuator and braking forces at the wheels are controlled in such a manner that the deviation between the determined behaviour index value and the target behaviour index value for the vehicle corresponding to the amount of operation of the operating member is reduced, thereby stabilizing the vehicle behaviour.
According to the first aspect of the present invention, if the vehicle is in the state of under-steer, then the steering actuator is controlled in such a manner that the deviation between the corrected behaviour index value for steering, instead of the behaviour index value, and the target behaviour index value is reduced, and the braking forces are controlled in such a manner that the deviation between the corrected behaviour index value for braking, instead of the behaviour index value, and the target behaviour index value is reduced. As the amount of under-steer increases, the deviation between the target behaviour index value and the corrected behaviour index value for steering decreases, and the deviation between the target behaviour index value and the corrected behaviour index value for braking increases.
According to the second aspect of the present invention, if the vehicle is in the state of under-steer, then the steering actuator is controlled in such a manner that the deviation between the corrected target behaviour index value for steering, instead of the target behaviour index value, and the behaviour index value is reduced, and the braking forces are controlled in such a manner that the deviation between the corrected target behaviour index value for braking, instead of the target behaviour index value, and the behaviour index value is reduced. As the amount of under-steer increases, the deviation between the corrected target behaviour index value for steering and the behaviour index value decreases, and the deviation between the corrected target behaviour index value for braking and the behaviour index value increases.
Therefore, according to the aspects of the present invention, as the degree of under-steer increases, the amount of control of the steering actuator for stabilizing the vehicle behaviour decreases, and the amount of control of the braking forces increases. Accordingly, in the state of under-steer, it is possible to prevent the steering angle from increasing excessively, and hence the vehicle behaviour can be stabilized. Moreover, the braking force for stabilizing the vehicle behaviour can be increased as the degree of under-steer rises, without having to provide complicated controls.
In the first aspect of the present invention, desirably, the vehicle wheel lateral slip angle is determined as the instability index value; the deviation between the target behaviour index value and the corrected behaviour index value for steering is taken as zero, when the magnitude of the vehicle wheel lateral slip angle is equal to or greater than a predetermined set value; and the predetermined set value for the magnitude of the vehicle wheel lateral slip angle is set at or below the maximum value of the magnitude of the wheel lateral slip angle which maintains a linear region in which the wheel lateral slip angle is proportional to the cornering force.
In the second aspect of the present invention, desirably, the vehicle wheel lateral slip angle is determined as the instability index value; the deviation between the behaviour index value and the corrected target behaviour index value for steering is taken as zero, when the magnitude of the vehicle wheel lateral slip angle is equal to or greater than a predetermined set value; and the predetermined set value for the magnitude of the vehicle wheel lateral slip angle is set at or below the maximum value of the magnitude of the wheel lateral slip angle which maintains a linear region in which the wheel lateral slip angle is proportional to the cornering force.
Thereby, it is possible to prevent the steering angle from increasing excessively in the case of under-steer, and hence the linear region, in which the lateral slip angle is proportional to the cornering force at the wheels, can be maintained, and the vehicle behaviour can be effectively prevented from becoming unstable by control of the braking forces.
Moreover, in the first aspect of the present invention, desirably, the deviation between the target behaviour index value and the corrected behaviour index value for braking is taken as zero, when the magnitude of the vehicle wheel lateral slip angle is zero.
Furthermore, in the second aspect of the present invention, desirably, the deviation between the behaviour index value and the corrected target behaviour index value for braking is taken as zero, when the magnitude of the vehicle wheel lateral slip angle is zero.
Thereby, it becomes possible to stabilize vehicle behaviour simply by controlling the steering angle, when the lateral slipping state of the wheels terminates, and hence unnecessary controls can be eliminated and the controls can be simplified.
According to the present invention, it is possible to provide a vehicle attitude control apparatus whereby stabilization of vehicle behaviour can be achieved in a vehicle which is in a state of under-steer, by controlling both the steering angle and the braking forces in an integrated manner, without needing to provide a complex control system.